Petroleum fuels such as residual fuel oils contain large amounts of impurities which result in corrosive deposits in the equipment. For example, crude oil usually contains 1-500 ppm of vanadium in the form of a porphyrin complex depending on the source. Because of its origin as a concentrate from the refining process, residual oil contains several times more vanadium than the crude from which it was derived. The combustion of these vanadium-containing fuels produces very corrosive deposits which can destroy a metal part, such as a gas turbine blade, in a matter of hours.
The presence of sodium in fuel can also have catastrophic consequences. For example, in maritime use the sodium level can be increased because of the introduction of sodium chloride through the air intake and contamination of the fuel by sea water. During combustion, the sodium can react with sulfur in the fuel to form a sulfate which is deposited in turbine parts.
Overbased detergents, e.g., overbased alkaline metal or alkaline-earth metal compounds, are well known additives for lubricating oil compositions and petroleum fuels. These detergents perform a variety of functions including anti-corrosion, deposit control, acid scavenger functions and in general comprise overbased metal compounds complexed with an organic dispersant. For example, overbased magnesium compounds complexed with sulfonate and carboxylate dispersants, have long been used as anti-corrosion and acidic neutralization additives for lubricating oils and greases, anti-corrosion and acidic neutralization additives during the combustion of fuels such as residual fuel, pulverized sulfur-containing coal, corrosion inhibitors in fuels containing vanadium etc. The addition of overbased magnesium detergents to, for example, boiler fuels or gas turbine fuels, is known to reduce corrosion, presumably by forming magnesium complexes with the vanadium or sodium.
Overbased metal detergents are also added to lubricating oils to prevent or remove deposits of oil-insoluble sludge, varnish, carbon and lead compounds which otherwise form on internal combustion engine parts and for combating severe rust conditions which may be encountered during shipping or storage of machinery or exposure to out-door weather. Detergent additives for automotive and diesel engine oils also react chemically with the highly acidic by-products of combustion that find their way into the lubricating oil system.
Obviously the dispersion must be stable during storage and the overbased metal must stay well dispersed in the lubricant or fuel. A variety of parameters will affect the stability and activity of these dispersions such as the dispersants and carriers employed, particle size of the solid components, and the relationship between metal and dispersant. The process by which the overbased metal compounds and complexes are prepared will greatly influence the actual physical make up and properties of the overbased metal dispersion, impacting particle size and distribution of the metal compound throughout the dispersion, the viscosity and stability of the dispersion, the amount of the metal within the dispersion etc.
Overbased metal additives, for example, overbased MgO dispersions, are typically added as a dispersion in an appropriate carrier, often in a high boiling liquid hydrocarbon. Part of the rationale for supplying MgO dispersions in high boiling carriers, i.e., carriers with boiling points over 280° C., typically much over 300° C. is due to the manner in which the dispersions are made. For example, overbased stable MgO dispersions with fine particle sizes and good flowability are typically produced, even when starting with MgO as a starting material, through thermal decomposition of Mg(OH)2 or Mg(OH)2 derived intermediates which require high temperature (300-350° C.). Thus, the use of high boiling point solvents as carriers is dictated by practical processing considerations.
U.S. Pat. No. 4,163,728, discloses stable, fluid magnesium-containing dispersions prepared by the high temperature decomposition of magnesium salts of carboxylic acids to MgO in a dispersant-containing fluid. In the process, Mg(OH)2, an organic carboxylic acid or sulfonic acid surfactant such as naphthenic acid, acetic acid and water are heated in a high boiling hydrocarbon to temperatures up to 350° C., which is above the decomposition point of magnesium acetate, 323° C. It is believed that magnesium acetate is formed in situ and decomposes at the high temperatures used. Water is also removed at the elevated temperatures.
U.S. Pat. No. 4,293,429, discloses a variation of U.S. Pat. No. 4,163,728 which begins with MgO instead of Mg(OH)2. In the process, the bulk MgO is converted to magnesium acetate which forms suspended MgO particles of less than 5 microns, and preferably less that 1 micron. Thus, the coarse MgO particles are converted into a dispersion of stabilized micro MgO particulates. It is also disclosed that similar processes using lower temperatures fail to provide the fine particle size MgO.
U.S. Pat. No. 4,056,479, discloses a fuel additive for reducing sediment in vanadium-containing fuels comprising a magnesium-alkoxide-carbonate complex in combination with an oil soluble sulfonate and a carboxylate and/or phenate dispersing agent. While the product has a magnesium content of about 12.5% to about 14.6%, it also tends to have undesirably high viscosities.
U.S. Pat. No. 4,129,589, discloses a process for preparing an over-based oil-soluble magnesium salt of a sulfonic acid by contacting carbon dioxide gas with a mixture comprising an oil-soluble magnesium salt of a sulfonic acid, magnesium oxide, water, and a promoter system comprising a carboxylic acid of 1 to 5 carbons in an inert solvent for lowering the viscosity of said mixture to facilitate mixing. The products of U.S. Pat. No. 4,129,589 had acceptably low viscosity but the magnesium content was typically 9-10% and no more than 14%.
U.S. Pat. No. 6,197,075, discloses an overbased magnesium sulfonate, carboxylate or phenate product containing at least 14% and up to about 18% by weight of magnesium, and a succinic anhydride and lower carboxylic acid co-promoter reaction product, useful as a deposit control additive for residual fuel oils and turbine fuels. The process for preparing the overbased magnesium product comprises contacting a mixture of i) a sulfonic acid, phenol or carboxylic acid or salt thereof, ii) a magnesium oxide, iii) a co-promoter comprising a lower carboxylic acid, a lower alcohol, a succinic anhydride and water, and iv) a solvent and/or oil, with an acidic gas such as carbon dioxide at 50° F. up to the reflux temperature of the mixture to overbase the reaction mixture.
The overbased metal compositions described above and elsewhere are best described as products by process as there is typically no simple chemical formula which adequately correlates to the essential material makeup and the physical properties of the product. Often, the molecular structures of the metal complexes are not fully known and are not a critical aspect of the invention. For example, two compositions containing compounds with the same chemical formula in the same amounts and differing only by the manner in which they were prepared can have very different physical properties.
While the use of high boiling solvents or carriers in the above processes can provide useful dispersions, there is the need for improved products and methods. For example, MgO dispersions with a higher magnesium content are desirable. However, attempts to modify known procedures to obtain overbased detergents with high metal content have met with unforeseen drawbacks including unacceptably high viscosities and gelling. Also, attempts to concentrate the dispersion by distillation to get higher Mg content must be carried out at very high temperatures or reduced pressure.
Co-pending application U.S. Ser. No. 13/167,127 describes one approach to obtaining free flowing MgO dispersions with high Mg content in high boiling carriers.
Alternate and flexible approaches to preparing MgO dispersions with high Mg content are still desirable. It has been found, as described herein, that performing some of the processing steps in the preparation of the MgO dispersions under increased pressure will allow one to prepare MgO dispersions in lower boiling solvents even when using the high temperatures required for conversion of the Mg(OH)2 or Mg(OH)2 derived intermediates.